1. Field of the Invention
This invention relates to the concentration and deactivation of actinide-containing materials from nuclear fuel cycles. More particularly, this invention relates to an electrochemical apparatus for concentrating and deactivating actinide-containing materials from nuclear fuel cycles. This invention further relates to a method for concentrating and deactivating actinide-containing materials from nuclear fuel cycles.
2. Description of Related Art
The processing of nuclear waste residue waste streams to reduce radio nuclide activity levels and matrix volume is a significant challenge which must be overcome to achieve nuclear stabilization and volume reduction so that geologic repositories will provide adequate storage volume. Although these nuclear waste residues contain fairly stable oxide forms, they are very dilute.
Residues containing actinides include graphite, pyrochemical salts, combustibles, incinerator ash, ceramic crucibles, plastic filters, and sand/slag crucibles. Currently, most of these wastes are stored and buried. It is known, however, that incinerator ash, sand and related materials can be treated with oxidative catalysts to reduce plutonium concentrations to a very low level.
Some oxides, such as TiO2 and SiO2, have been shown to attract actinide cations. See Morris, D. E., “Aqueous Electrochemical Mechanisms in Actinide Residue Processing”, Final Report to U.S. Department of Energy, LANL Project 59967, Sep. 30, 2000. In this report, a mediated electrochemical oxidation/reduction process (MEO/R) was used to achieve nuclear stabilization and volume reduction. In particular, sorption reactions of UO22+ and Eu3+ on SiO2 and TiO2 and several aluminosilicate minerals were investigated. In this electrochemical process, anion clusters of SiO2 and TiO2 are formed in an aqueous solution which adsorb the nuclear cations to form precipitates. Thus, the nuclear wastes are entrained in the metal oxides and form stable suspensions with high solids concentrations. However, disadvantageously, this method requires the use of a fine powder capture process and it is difficult to separate the metal oxide and the nuclear concentrates.
Known water purification methods include distillation, ion-exchange, carbon adsorption, filtration, ultrafiltration, reverse osmosis, electrodeionization, ultraviolet radiation and combinations thereof. However, each of these methods has shortcomings. Distillation cannot remove some volatile organics and it consumes large amounts of energy. In the ion-exchange process, water is percolated through bead-like spherical resin materials. However, the resin materials need to be regenerated and changed frequently. In addition, this method does not effectively remove particles, pyrogens, or bacteria. The carbon adsorption process can remove dissolved organics and chlorine with long life and high capacity; however, fine carbon particles are generated during the process due to corrosion. Micropore membrane filtration, a high cost process, removes all particles and microorganisms greater than the pore size of the membrane; however, it cannot remove dissolved inorganics, pyrogens or colloids. The ultrafilter is a tough, thin, selectively permeable membrane that retains most macromolecules above a certain size, including colloids, microorganisms, and pyrogens; however, it will not remove dissolved organics. Reverse osmosis is the most economical method for removing 90 to 99% of all contaminants. Reverse osmosis membranes are capable of rejecting all particles, bacteria, and organics; however, the flow rate or productivity is low. Electrodeionization is a combination of electrodialysis and ion-exchange, resulting in a process which effectively deionizes water while the ion-exchange resins are continuously regenerated by the electric current; however, this method requires pre-purification to remove powders and ash materials.
FIG. 1 is a diagram showing a capacitive deionization process with carbon aerogel electrodes. In this process, salt water is introduced into the cell, the negative electrode (anode) 11 adsorbs positive ions 13 and the positive electrode (cathode) 12 adsorbs negative ions 14. When the cell is charged, pure water is obtained, and when the cell is discharged, concentrated salt water is removed. To achieve this result, pulsed electrical power at voltages from 1.2V to 0V is used for different time periods depending on the concentration of the salt water and the activity of the activated carbon. The more accessible surface area the electrode has, the more ions that can be stored. The main problem with this method is that the electrosorption capacity (salt removal) decreases with cycle life. Most of the capacity loss can be recovered by periodic reversing of the electrode polarization. However, the interface between the active carbon and the aerogel diminishes, reducing the actual electrode active area. That is, the carbon particles will no longer contact each other and, ultimately, will leach out.